1. Field of the Invention
The present invention is directed to solid state fabrication techniques and, more particularly, to techniques used to produce three-dimensional, doped films particularly useful in the construction of solid state memory devices.
2. Description of the Background
Memory cells, such as those of dynamic random access memories (DRAM), are comprised of two main components: a field-effect transistor (FET) and a capacitor. In memory cells utilizing a conventional planar capacitor, far more chip surface area is dedicated to the planar capacitor than to the FET. Wordlines are generally etched from a polysilicon-1 layer. A doped region of silicon substrate functions as the lower (storage-node) capacitor plate while a doped polysilicon-2 layer generally functions as the upper capacitor plate (cell plate). Although planar capacitors have generally proven adequate for use in memory chips up to the one-megabyte level, they are considered to be unusable for more advanced memory generations.
As component density in memory chips has increased, the shrinkage of cell capacitor size has resulted in a number of problems. Firstly, the alpha-particle component of normal background radiation can generate hole-electron pairs in the silicon substrate, which can be collected by the lower capacitor plate. The phenomena will cause a charge stored within the affected cell capacitor to rapidly dissipate, resulting in a "soft" error. Secondly, the sense amplifier differential signal is reduced. That aggravates noise sensitivity and makes it more difficult to design column sense-amplifiers having appropriate signal selectivity. Thirdly, as cell capacitor size is decreased, the smaller charge stored within the cell leaks to an unusable level sooner, which necessitates more frequent interruptions for refresh overhead.
As a result of the problems associated with the use of planar capacitors for high-density memories, manufacturers of, for example, 4-megabyte DRAMs are utilizing cell designs based on non-planar capacitors. Two basic non-planar capacitor designs are currently in use: the trench capacitor and the stacked capacitor. Both types of non-planar capacitors typically require a considerably greater number of masking, deposition, and etching steps for their manufacture than does a planar capacitor.
In a trench capacitor, charge is stored primarily vertically, as opposed to horizontally in a planar capacitor. Because trench capacitors are fabricated in trenches which are etched in the substrate, some trench capacitor structures can be susceptible to soft errors. In addition, there are several other problems inherent in the trench design. One problem is that of trench-to-trench charge leakage caused by the parasitic transistor effect between adjacent trenches. Another problem is cell storage node-to-substrate leakage attributable to single crystal defects which are induced by stress associated with the trench structure. Yet another problem is the difficulty of completely cleaning the trenches during the fabrication process. Failure to completely clean a trench will generally result in a defective cell.
The stacked capacitor design, on the other hand, has proven somewhat more reliable and easier to fabricate than the trench design. However, in the stacked capacitor design, the layer of material which forms the storage node is in contact with the substrate. Subsequent processing steps tend to cause outdiffusion of the dopant which adversely affects the diode junction profile as well as the threshold voltage for the access transistor. Thus, the need exists for a method of controlling the outdiffusion from a doped three-dimensional film.